118 THE BIOLOGICAL PROBLEM OF TO-DA Y 



of the blastopore. When fusion of the lips took 

 place the normal lip united with the rim of the 

 protruding hypertrophied lip. As a result of this 

 the notochord and the nerve plate came to arise, not 

 from the usual set of cells, but from those cells that, 

 by the abnormal condition, had come to lie in the 

 place for the notochord. The protruding cells, 

 which normally would have developed into noto- 

 chord and nerve plate, grew into a simple fold of the 

 external skin. 



Moreover, it is well known in pathology that 

 mucous membranes may lose their proper char- 

 acter and assume the qualities and aspect of the 

 external skin, when, as in cases of prolapse, fistula, 

 etc., they have been exposed for some time to the 

 air. 



The relations of different parts to each other and 

 to the whole are known as correlations. Correla- 

 tion exists in all the stages of the development 

 of an organism, sometimes in one way, sometimes 

 in another. One must note very carefully that 

 Weismann's doctrine of determinants, according to 

 which all that happens in development follows a 

 prearranged plan, is entirely in opposition to this 

 correlative character of the changes that occur 

 during development. 



Here I shall give a few quotations from botanical 

 and zoological writers : 



1 If the stem of a plant be cut so that it retains 

 its roots, but is deprived of leaves and shoots, then 

 the adventitious buds will produce new leaves 

 and shoots. If, however, the stem be cut so as to 



